This invention relates to methods for forming polyesters and polyester copolymers from cyclic oligomeric esters.
The ring-opening polymerization of cyclic oligomers containing ester linkages is a convenient way of preparing high molecular weight polyesters. Although polyesters are thermoplastics, and can be melt processed even as high molecular weight polymers, the polymerization of the cyclic oligomers offers the possibility of conducting molding or other melt processing operations simultaneously with the polymerization. The oligomers melt to form relatively low viscosity fluids that can be easily pumped and/or used to impregnate a variety of reinforcing materials. Therefore, using cyclic oligomers provides a means by which a high molecular weight polymer can be processed much like many thermosetting polymer systems.
The ring-opening polymerization is conducted in the presence of a catalyst in order to obtain commercially reasonable cycle times. A variety of basic amine, weak carboxylic acid salts, sulfonic acids, phosphonium tetraphenyl borates, tin compounds, titanium compounds and other metal compounds have been used. 1,3-dichloro-1,1,3,3-tetrabutyldistannoxane has been reported as a catalyst for the solid-state polymerization of a macrocyclic PET dimer, producing a polymer with a somewhat low weight average molecular weight in the range of ˜22,000–36,000. See, e.g., Nagahata et al., “Solid-State Thermal Polymerization of Macrocyclic Ethylene Terephthalate Dimer Using Various Transesterification Catalysts”, J. Polym. Sci. Part A: Polym. Chem, Vol. 38, 3360–3368 (2000).
The catalyst desirably fulfills several needs. First, it must be an active and efficient catalyst. Second, the catalyst is preferably a liquid under polymerization conditions, so it can be handled and metered easily. Third, the catalyst and its residues desirably have little or no activity in promoting reactions that significantly degrade the molecular weight of the polymer during thermal processing operations.